1. Field of the Invention
The present invention relates to a driving method and a driving circuit for a color liquid crystal display and more particularly to the driving method and the driving circuit for driving the color liquid crystal display based on a gamma compensated video signal.
The present application claims the Convention Priority of Japanese Patent Application No. Hei11-316873 filed on Nov. 8, 1999, which is hereby incorporated by reference.
2. Description of the Related Art
FIG. 19 is a block diagram showing a conventional electric configuration of a driving circuit of an analog circuit configuration of a color liquid crystal display 1.
The color liquid crystal display 1 is a liquid crystal display of an active matrix driving type using a TFT (Thin Film Transistor) as a switching element, in which intersection points of plural scanning electrodes (gate lines) provided at predetermined intervals in a row direction and plural data electrodes (source lines) provided at predetermined intervals in a column direction are used as pixels, for each pixel, a liquid cell of a equivalent capacitive load, a TFT for driving a corresponding liquid crystal cell, a capacitor for keeping data charges during one vertical synchronous period are arranged, a data red signal, a data green signal and a data blue signal generated based on a video red signal SR, a video green signal SG, a video blue signal SB, are applied to the data electrode and a scanning signal generated based on a horizontal synchronous signal SH and a vertical synchronous signal SV is applied to a scanning electrode, and then a color character, a color image and a like are displayed. In addition, the color liquid crystal display 1 is a normal white type having a high transmittance when no voltage is applied.
Further, the driving circuit of the color liquid crystal display 1 is mainly provided with clamp circuit 21 to clamp circuit 23, a reference voltage generating circuit 3, gamma compensating circuit 41 to gamma compensating circuit 43, polarity inverting circuit 51 to polarity inverting circuit 53, video amplifier 61 to video amplifier 63, a timing generating circuit 7, a data electrode driving circuit 8 and a scanning electrode driving circuit 9.
Clamp circuit 21 to clamp circuit 23 execute a clamp fixing (direct current refreshing) a level of a top or a back porch of the horizontal synchronous signal SH of the video red signal SR, the video green signal SG and the video blue signal SB supplied from outside to a black level and output a video red signal SRC, a video green signal SGC and a video blue signal SBC.
The reference voltage generating circuit 3 a generates a reference voltage VL, a reference voltage VM, a reference voltage VH used to gamma compensate the video red signal SRC, the video green signal SGC and the video blue signal SBC and supplies the video red signal SRC, the video green signal SGC and the video blue signal SBC to gamma compensating circuit 41 to gamma compensating circuit 43. Gamma compensating circuit 41 to gamma compensating circuit 43, based on the reference voltage VL, the reference voltage VM and the reference voltage VH supplied from the reference voltage generating circuit 3, give a gradient to the video red signal SRC, the video green signal SGC and the video blue signal SBC by gamma compensating the video red signal SRC, the video green signal SGC and the video blue signal SBC and output them as the video red light SRG, the video green light SGG and the video blue light SBG.
Here, the gamma compensation will be explained. For example, when a logarithm value of a luminance originally provided for a subject such as a view and a person taken by a video camera is set to a horizontal axis and a logarithm value of a luminance of a reproduced image displayed on a display by a video signal from the video camera is set to a vertical axis and then an inclination angle of a reproducing characteristic curve is set to θ, tan θ is called a gamma (γ) . When the luminance of the subject is reproduced on the display with fidelity, namely, when an input (horizontal axis) increases or decreases by one and also an output (vertical axis) increases or decreases by one, the inclination angle of the reproducing characteristic curve is a straight line having an inclination angle of 45°, tan 45°=1 and then the gamma becomes 1. Therefore, in order to reproduce the luminance of the subject with fidelity, it is necessary to set a gamma of a whole system including taking the subject by the video camera though reproducing an image on the display to gamma=1.
However, an image pickup element such as CCD (Charge Coupled Device), a CRT (Cathode Ray Tube) display or a like making up a video camera has a peculiar gamma. A gamma of the CCD is 1 and a gamma of the CRT display is about 2.2.
Therefore, it is necessary to compensate a video signal in order to obtain a reproduced image of good gradation by setting gamma=1 as a whole system, and this is called gamma compensation. Generally, the gamma compensation is applied to the video signal so as to be suitable to a gamma characteristic of the CRT display.
Polarity inverting circuit 51 to polarity inverting circuit 53, in order to alternately drive the color liquid crystal display 1, invert respective polarities of the video red light SRG, the video green light SGG and the video blue light SBG and output them. Video amplifier 61 to video amplifier 63 amplify the video red light SRG, the video green light SGGand video blue light SBG which are polarity-inverted to a level until the color liquid crystal display 1 can be driven. The timing generating circuit 7, based on the horizontal synchronous signal SH and the vertical synchronous signal SV supplied from outside, generates a horizontal scanning pulse PH and a verticality scanning pulse PV and supplies the horizontal scanning pulse PH and the verticality scanning pulse PV to the data electrode driving circuit 8 and the scanning electrode driving circuit 9. The data electrode driving circuit 8 generates a data red signal, a data green signal, a data blue signal from the video red light SRG, the video green light SGG and the video blue light SBG which are amplified and polarity-inverted and applies the data red signal, the data green signal and the data blue signal to corresponding data electrodes in the color liquid crystal display 1 at a timing of the horizontal scanning pulse PH supplied from the timing generating circuit 7.
The scanning electrode driving circuit 9 generates a scanning signal and supplies the scanning signal to a corresponding scanning electrode in the color liquid crystal display 1 at a timing of the vertical scanning pulse PV supplied from the timing generating circuit 7.
Further, FIG. 20 is a block diagram showing a second conventional electric configuration of a driving circuit of a digital circuit configuration for the color liquid crystal display 1.
The driving circuit for the color liquid crystal display 1 is mainly provided with a controlling circuit 11, a gradation power supply circuit 12, a data electrode driving circuit 13 and a scanning electrode driving circuit 14.
The controlling circuit 11 is, for example, an ASIC (Application Specific Integrated Circuit), supplies red data DR of six bits, green data DG of six bits and blue data DB of six bits supplied from outside to the data electrode driving circuit 13 and generates a horizontal scanning pulse PH, a vertical scanning pulse PV and a polarity inverting pulse POL for alternately driving the color liquid crystal display 1 and supplies them to the data electrode driving circuit 13 and the scanning electrode driving circuit 14. The gradation power supply circuit 12, as shown in FIG. 21, is provided with resistor 151 to resistor 1511 connected longitudinally between a reference voltage VREF and ground and voltage follower 161 to voltage follower 169 connected with connection points of resistors adjacent to respective input terminals, and applies buffer to a gradation voltage V0 to a gradation voltage V9 set for the gamma compensation and appearing at connection points of adjacent resistors and supplies gradation voltage V0 to gradation voltage V9 to the data electrode driving circuit 13.
The data electrode driving circuit 13, as shown in FIG. 21, is mainly provided with a multiplexer (MPX) 17, a DAC 18 and voltage follower 191 to voltage follower 19384. In addition, a real data electrode driving circuit is provided with a shift register, a data register, a latch and a level shifter at a front step of the DAC 18, however, these elements and operations are not directly related with features of the present invention, therefore, explanations are omitted in this specification and they are not shown.
The multiplexer MPX 17 switches a group of gradation voltage V0 to gradation voltage V4 and a group of gradation voltage V5 to gradation voltage V9 among gradation voltage V0 to gradation voltage V9 supplied from the gradation power supply circuit 12, based on the polarity inverting pulse POL supplied from the controlling circuit 11 and supplies one of the groups to the DAC 18. The DAC 18 applies the gamma compensation to the red data DR of six bits, the green data DG of six bits and the blue data DB of six bits supplied from the controlling circuit 11, converts the red data DR, the green data DG and the blue data DB into an analog data red signal, an analog green signal and an analog blue signal and supplies the analog data red signal, the analog green signal and the analog blue signal to voltage follower 191 to voltage follower 19384, based on the group of gradation voltage V0 to gradation voltage V4 and the group of gradation voltage V5 to gradation voltage V9. Voltage follower 191 to voltage follower 19384 apply buffer to the analog data red signal, the analog data green signal and the analog data blue signal supplied from the DAC 18 and apply these data signals to corresponding data electrodes in the color liquid crystal display 1.
The scanning electrode driving circuit 14 sequentially generates scanning signals and sequentially applies the scanning signals to corresponding scanning electrodes in the color liquid crystal display 1 at a timing of the vertical scanning pulse PV supplied from the timing generating circuit 7.
Now, in the driving circuit for the color liquid crystal display 1 of the first conventional example, the gamma compensation is applied to the video red signal SRC, the video green signal SGC and the video blue signal SBC based on the common reference voltage VL, the common reference voltage VM, the common reference voltage VH, so that the gamma characteristic of the CRT display (gamma is about 2.2) is suitable for the video red signal SRC, the video green signal SGC and the video blue signal SBC.
Further, in the driving circuit for the color liquid crystal display 1 of the second conventional example, the gamma compensation is applied to the red data DR, the green data DG and the blue data DB based on the common gradation reference voltage V0 to the common reference voltage V4 and common gradation reference voltage V5 to common gamma reference voltage V9 so that the gamma characteristic of the CRT display (gamma is about 2.2) is suitable for the red data DR, the green data DG and the blue data DB.
However, a color liquid crystal display 1 has a gamma characteristic different from that of a CRT display, a characteristic curve of a transmittance T for an applied voltage V (a V-T characteristic curve) is not linear, and particularly, the transmittance hardly changes against a change of the applied voltage near a black level. Further, since the V-T characteristic curve of the color liquid crystal display, as shown in FIG. 22, is different for each of a red (curve a), a green (curve b) and a blue (curve c), a characteristic curve of the luminance (an output) for the gradation (an input), as shown in FIG. 23, is different for each of the red (curve a), the green (curve b) and the blue (curve c) . In FIG. 23, the luminance is a relative luminance in which the gamma compensation is applied to the video signal so as to be suitable to a gamma characteristic of a CRT display (about 2.2 gamma) in the gamma compensating circuit.
Accordingly, in the conventional gamma compensation common with the red, the green and the blue and making suitable to the gamma characteristic of the CRT display (about 2.2 gamma), for example, in a case of the V-T characteristic curve shown in FIG. 22, a transmittance is set to 100% when an applied voltage is 1.7 V, namely, a white level is set. However, particularly in the green (curve b), a white level is set at transmittance of 80%, therefore, it is impossible to carry out an optimal gamma compensation and then it is impossible to obtain a reproduced image of a good gradation. As a result, there a disadvantage in that it is impossible to meet a recent need of a high video quality.
Further, recently, in order to meet the need of the high video quality, a color liquid crystal display having a high transmittance is developed, and FIG. 24 shows an example of a V-T characteristic curve of a color liquid crystal display having such a high transmittance characteristic red (curve a), green (curve b), blue (curve c)) . In such the V-T characteristic curve, each of red (curve a), green (curve b) and blue (curve c) has a transmittance of 100%, namely, each best luminance is too different, therefore, there is a problem in that the color liquid crystal display 1 cannot be used since it is impossible to deal with gamma characteristics of the conventional gamma compensation which are used in common with red, green and blue.
Furthermore, as above described, in the first conventional example and the second conventional example of a driving circuit for the color liquid crystal display, gamma compensation is applied based on common reference voltage VL, common reference voltage VM and common reference voltage VH or a common group of gradation voltage V0 to gradation voltage V4 and a common group of gradation voltage V5 to gradation voltage V9, therefore, there is a problem in that, though a gradation batter occurs in which gradation change is not displayed on a display as luminance changes, the gradation batter can not be removed.